One of the interesting presentations at Alberta Conservation Tillage Society's annual meeting was given by Dr. Wayne Lindwall, acting director of the Lethbridge Research Station. He cited many studies done in the western Canadian prairies showing the long term effects of direct seeding. Knowing some of the key findings will give direct seeders a good idea of what changes to watch for in their own system.
Most direct seeders know that direct seeding protects their soil from erosion by keeping residue on the soil surface. And most probably don't need research plots to tell them that direct seeding is one of the best crop production systems to keep residue on the surface of their soils. However one of the disadvantages of direct seeding is that it produces high amounts of small soil clods or aggregates which are erodible. There is no mechanical means of creating larger non erodible clods as in a tillage application. Residue is the best defense against erosion but in a direct seeding system it is imperative to keep the residue in place. If you are going to for example burn stubble be aware of the danger of erosion and get a canopy over that soil as soon as possible. Other studies did show, though, that the aggregates under direct seeding were more stable.
In terms of how soil water contents are affected by direct seeding most of the studies showed an increase in water content in the top layer of soil after 7 or 8 years of direct seeding. For example at Indian Head in the top 120 cm the moisture content went up an average of 6%.
Lindwall sums up soil temperature results by saying , "Direct seeded soil is colder because it is wetter and it is wetter because it is colder." Surface residue keeps the soil cooler and reduces evaporative loss of water. Temperature differences between tillage systems were generally only found in the seed zone of the soil profile. They showed that conventional tillage was warmer during the day but the same or cooler at night as compared to direct seeding.
Organic matter content as measured by organic carbon is another change that direct seeders can watch for in their soil. Research shows that organic carbon increased under direct seeding but the changes took 5-10 years or longer to become evident. Soils which are already higher in organic matter may not change as much as low organic soils. Also, the changes will be concentrated on the top of the soil profiles that is where the residues are being returned. For example, Con Campbell found a 10.5% increase in organic carbon in the top 6 in. of a fine sandy loam near Swift Current after 11 years of continuous direct seeded wheat.
Another interesting finding is that studies show a more active microbial population under direct seeding. In fact one study concluded that 2,4-D was degraded faster under conservation till plots because of increased biological and microbial activity.
These are some of the long term changes observed under direct seeding.